<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>第一个css案例</title>
    <style>
        #box{
            width: 400px;
            height: 280px;
            margin: 0px auto;
            background-color: lightcyan;
            border: 1px solid #ccc;
            text-align: center;
            line-height: 280px;
        }
    </style>
</head>
<body>
<div id="box">
    我是DIV的内容
</div>
</body>
</html>